1. Field of the Invention
The present invention relates to a driving mechanism for a photographic lens adapted to move the photographic lens back and forth in parallel to an optical axis for power varying or focusing.
2. Description of the Related Art
As photographic cameras become more popular, marketing trends require that cameras be more and more compact, thin, and lightweight and simultaneously contain a power varying mechanism such as a zoom mechanism or focus change-over mechanism. The power varying mechanism functions to move the photographic lens back and forth in a direction parallel to an optical axis, for example, to move the photographic lens forward with respect to a camera body for telephoto and move the photographic lens back toward the camera body for wide-angle or standard photo. Thus, a lens barrel holding the photographic lens is moved back and forth relative to the camera body in parallel to the optical axis.
In order to assure that the photographic lens can be moved within a given range to obtain a desired magnification while making the camera as thin as possible, the lens barrel assembly must be constructed with four stages of which three stages can be successively moved. More specifically, the lens barrel assembly must be constructed so that a first driving lens barrel is movable back and forth relative to a stationary lens barrel, a second driving lens barrel is movable back and forth relative to the first driving lens barrel, and a movable lens barrel holding a part of the photographic lens is movable back and forth relative to the second driving lens barrel. By adopting such arrangement, it is possible to limit a length of the lens barrel assembly in the direction of the optical axis approximately to a length of the stationary lens barrel and to obtain a moving range of the photographic lens approximately corresponding to three times the length of the stationary lens barrel. The respective stages of the lens barrel assembly must keep the photographic optical system in a predetermined optical relationship during their movement.
A driving mechanism to move the lens barrel assembly back and forth utilizing a cam barrel is also well-known and the above-mentioned arrangement can be adopted in such a driving mechanism. However, a length of the cam will be unacceptably increased to assure an adequate moving range of the lens barrel assembly while making the camera body as thin as possible. Consequently, a cam slit must be formed in the lens barrel assembly over its fill length, resulting in an apprehension that the strength of the lens barrel assembly might be unacceptably decreased. Particularly, with the photographic lens employing a mechanism adapted to perform the operation of power varying and focusing from a single power source for the economical reasons, the cam's length is increased resulting in a decrease in the cam's strength. To avoid such an inconvenience, a cam groove having a bottom, instead of a cam in the form of a through-hole, has been adopted as the cam formed in the lens barrel assembly.
However, the driving mechanism formed with the previously-described cam groove having the bottom suffers from the following problem. With the conventional mechanism utilizing the cam slit formed in the lens barrel assembly to drive the photographic lens, a pin member interlocked with the cam slit is operatively associated with the cam slits of two lens barrel stages, one serving to guide a movement in parallel to the optical axis and the other serving to regulate a distance of this movement. With the driving mechanism adopting the cam in the form of a groove having a bottom, on the other hand, it is impossible for the pin member to extend through the lens barrel assembly and therefore, in addition to the lens barrel assembly formed with the cam groove, a driving ring member or the like must be provided to control a direction as well as a distance of the pin member's movement.
It is also well-known from Japanese Patent Application Disclosure Gazette No. 1995-128567 to move one driving lens barrel back and forth in parallel to the optical axis without rotation around the optical axis and to rotate the other lens barrel about the optical axis to thereby move it back and forth in parallel to the optical axis so that a front lens group holding frame may be moved back and forth in parallel to the optical axis as the other lens barrel rotates.
In the case of a triple-stage zoom lens barrel assembly described in Japanese Patent Application Disclosure Gazette No. 1995-128567, a stationary lens barrel comprises inner and outer barrels between which a rotatable barrel is disposed. Such barrel assembly of double structure correspondingly enlarges the outer diameter of the lens barrel assembly and limits the possibility of making the camera compact.
In view of the problem described above, the applicant of the present application has already developed and proposed a driving mechanism for a photographic lens that is improved so that additional use of parts such as a rotatable barrel or a driving ring as well as adoption of the lens barrel assembly of dual-structure can be avoided while still realizing a more compact camera (Japanese Patent Application No. 1995-295942 and U.S. patent application Ser. No. 08/733,401). The driving mechanism for photographic lens disclosed in this prior patent application is characterized in that a movable lens barrel is moved back and forth as a second driving lens barrel, which telescopically receives therein the movable lens barrel, is rotated. The second driving lens barrel is moved back and forth as a first driving lens barrel, which telescopically receives therein the second driving lens barrel, and is rotated relative to the second driving lens barrel. The first driving lens barrel is moved back and forth relative to a stationary lens barrel, which telescopically receives therein the first driving lens barrel, as the first driving lens barrel is rotated. A second linearly movable member that is provided to prevent the movable lens barrel from being rotated, is moved back and forth together with the second driving lens barrel. A first linearly movable member that is provided to prevent the second linearly movable member from being rotated, is moved back and forth together with the first driving lens barrel.
However, the driving mechanism for a photographic lens disclosed in the above-mentioned patent application adopts an arrangement such that the second linearly movable member is guided by the first linearly movable member and, as a result, may suffer from the following problem. Specifically, the first linearly movable member is combined with the second linearly movable member so that their portions to guide and their portions to be guided define together a common cylinder. More specifically, the respective linearly movable members basically comprise cylindrical bodies and each of them is circumferentially divided so as to form an appropriate number of openings and keys. The keys of the first linearly movable member are received by the respective openings of the second linearly movable member. Thus, a distal end of the first linearly movable member is circumferentially discontinuous due to the presence of the openings and the keys are supported on a proximal end of the first linearly movable member in a cantilever fashion. Additionally, both the first linearly movable member and the second linearly movable member must be disposed within the lens barrel assembly and, for this reason, their thickness must be as thin as possible. In consequence, the first linearly movable member is problematic due to its lack of strength, and there is an apprehension that this member might be damaged due to fatigue.
Further, the first linearly movable member is operatively associated with a rear end of the first driving lens barrel and with the stationary lens barrel so that the first linearly movable member is reliably prevented from being rotated. Such arrangement necessarily results in a situation that the keys of the first linearly movable member lie adjacent the plane of photographic film. The keys lie inside the lens barrel assembly and therefore adjacent the optical axis. Thus, there is an apprehension that the keys of the first linearly movable member might cause an undesirable flare phenomenon.